Wide band-gap semiconducting crystals such as silicon carbide (SiC), group III-nitrides e.g. gallium nitride (GaN) and aluminum nitride (AlN); offer several attractive electrical and physical properties for fast switching power devices and optoelectronic devices. These wide band gap semiconductors and their alloys also differentiate themselves from other important semiconductors, such as silicon and gallium arsenide, by the fact that they cannot at present be directly grown from a melt or a liquid solution under practically and economically interesting conditions. Instead, ingots of SiC, GaN or AlN are usually grown from the vapor phase, by the epitaxial deposition of a supersaturated vapor flux onto a seed crystal.
In the case of SiC, the first method developed to produce semiconductor grade SiC crystals (also called ingots or boules) of diameter and length sufficient to manufacture wafers for device applications, is the sublimation method, also known as physical vapor transport (PVT). The core concept of this method has been introduced by Lely in 1955 (Berichte der Deutschen Keramische, Ges. 32-8 p. 229 (1955) and has been modified in 1978 by Tairov and Tsvetkov (J. Cryst. Growth 52, p. 146 (1981) to produce consistent semiconductor grade SiC crystals where key properties such as the polytype and the growth rate of the crystal can be controlled. Briefly, the method is based on the use of a sealed crucible in which a temperature gradient is established between a high temperature zone, where a solid source material such as a SiC powder is sublimed, and a lower temperature region in which the sublimed species crystallize on a seed crystal.
The sublimation method is at present also developed by different groups for the growth of AlN and GaN bulk crystals, while hydride vapor phase epitaxy and liquid phase techniques are also being investigated for the growth of bulk GaN crystals.
The sublimation method allows today the production of SiC wafers of diameters of 50 and up to 100 mm with entry quality and cost sufficient to enable industrial manufacturing of devices such as LEDs and Schottky diodes.
Despite these achievements, there are however some challenges and limits in the sublimation technique. For example, as long as no continuous feeding mechanism can be devised, the initial mass of the feedstock limits the duration of a continuous crystal growth process and thus the crystal length. One difficulty may, for example, be the need to control a changing sublimation rate and a drift of the sublimed species stoichiometry during growth. Instabilities in the source material supply and drifts of the temperature distribution in the source feedstock, for example, cause drifts of the growth rate and of the incorporation of doping species. If not properly controlled, such drifts tend to adversely affect the yield of the crystal growth process.
These challenges may be solved by further improvements of the sublimation process, and in the case of SiC, the capability of the technique to produce wafers on a relatively large scale, is an indication of its industrial potential.
An alternative industrially interesting technique, which does provide a continuous control of the source material supply together with the potential of growing long crystals from the vapor phase has been introduced in 1995 by U.S. Pat. No. 5,704,985. This technique is generically described as High Temperature Chemical Vapor Deposition (HTCVD) and differs from sealed PVT configuration by making use of an open hot-wall configuration offering an accurate control in the supply of the source and doping materials. In particular, at least one of the components of the grown material is continuously supplied in the form of a regulated gas flow and fed into a high temperature region through an inlet opening. Additionally, an exhaust is provided downstream of the crystallization region to control the gas flow along the growing crystal surfaces and exhaust the by-products resulting from the crystallization process. The technique may be described as Chemical Vapor Deposition (CVD) owing to its conceptual similarity with the CVD techniques used to grow epitaxial layers of 0.1 to 200 μm thickness. However, as taught in U.S. Pat. No. 5,704,985 and No. 6,048,398, in order to reach growth rates economically interesting for producing large bulk crystals, the HTCVD technique uses an order of magnitude higher source gases feed rates and several hundreds degrees higher temperatures than normal CVD processes.
For example, in a device similar to the one of the first figures of U.S. Pat. No. 5,704,985 (FIG. 1), in the specific case of SiC, by heating the seed crystal (13) to a temperature of 2250° C. and feeding via inlet 15 a gas mixture containing 0.3 L/min of silane and 0.1 L/min of ethylene diluted in a carrier gas, a growth rate of 0.5 mm/h is obtained.
However, when carrying out the method for several hours, it is experimentally observed that SiC also crystallizes around the seed crystal substrate (13), onto the holder (12) made for example of graphite, and on the exposed surfaces of the exhaust holes (14) in FIG. 1. On the surfaces in the immediate vicinity of the seed crystal (13), SiC crystallizes in a dense polycrystalline solid comprising mainly 6H and 15R polytypes. Further downstream in the exhaust holes 14, SiC crystallizes in somewhat less dense polycrystalline grains, often needle shaped and of the 3C polytype. The dense polycrystalline deposition can occur at a rate approximately twice that of the single crystal crystallization rate. Further downstream, as the temperatures decreases and the supersaturation increases, the less dense polycrystalline deposits grow even faster, eventually obstructing the gases outlet path within 2 to 4 hours. Once the exhaust path downstream of the seed crystal is sufficiently obstructed, a pressure difference rapidly builds up between the sources gases inlet 15 and the exhaust port 16. If the pressure differential is allowed to reach a few mbars, a rapid deterioration of the polytype and the structural quality of the single crystal occurs. The source gases can also start to flow along a path of higher conductance than the one of the obstructed exhaust 14, for example through any porous insulating material such as 15 in FIG. 1. The thermal properties of the insulating material are then rapidly deteriorating due to reaction with silicon, which forces the growth to be interrupted. Alternatively, when the exhaust path 14 becomes obstructed under conditions where the source gases are not allowed to find a path of higher conductance, a very rapid blocking of the gases inlet conduct takes place by polycrystalline silicon deposition. In this case, the growth also needs to be interrupted as no source gases can be supplied to the single crystal.
The parasitic deposition of polycrystalline solid phases thus leads to a catastrophic runaway of the system, forcing to terminate the growth process before a crystal of a desired length is produced.
A tentative solution to solve this problem has been presented in the PCT application WO 98/14644. In the example of SiC crystal growth, this application describes a device where the Si and C containing process gases are separated from the main heating element 7 in FIG. 2 by a thin inner cylinder 25. A blanketing inert gas is forced to flow between the main heating element and the inner cylinder, the inner cylinder ending at a distance approximately corresponding to the single crystal growth front. Downstream of the single crystal growth front, the blanketing gas guided along the walls of the main heating cylinder, is meant to prevent or substantially slow down deposition of polycrystalline SiC on the downstream inner walls and to slow down growth of polycrystalline SiC on the seed holder 13, so that the outlet path 31 remains free. A similar solution is presented in the European patent application no. 787,822 A1 where an inert blanketing gas flowing parallel to the process gases stream is presented for a device operating between 800 and 2500° C.
It has been found that this solution, as presented in or derived from these documents, does not solve the problem described above to an extent sufficient to grow SiC or other crystals of a length more than a few mm. Experiments using an inert blanketing gas, such as helium or argon, showed that too rapid polycrystalline deposition still occurred on the downstream regions of the single crystal growth front. When helium is used as blanketing gas, an even higher polycrystals growth rate is easily obtained, whereas the use of argon only pushes the deposition region a short distance downstream. This unexpected result can be explained by an additional flux of carbon carried by the blanketing gas when it passes along a graphite made uncoated heating element and by the differing thermal conductivity of the two considered gases. In a silicon rich exhaust gas mixture, any additional carbon supply leads to an increase of the downstream growth of polycrystalline SiC. A similar phenomenon is observed when using a heating element 7 coated with SiC. To circumvent this, it will be obvious to a person skilled in the art to use, as an improvement, a heating element and guide coated with a metal carbide such as for example TaC or NbC. Preferably the exposed surfaces will also have a low surface roughness to offer less nucleation sites to polycrystalline SiC. Under typical process conditions leading to a single crystal growth rate of 0.5 to 1 mm/h, it is however observed that such a design only leads to a further downstream location of the uncontrollable polycrystalline SiC deposition. This small improvement of the blocking time is not sufficient to continuously grow several cm long crystals.
In other prior art devices designed to grow SiC crystals where at least one component of the material to be grown is fed as a gas and the by-products of the process are exhausted via an opening in the crucible, no solution to parasitic deposition of the polycrystalline form of the material to be grown is mentioned. For example, European patent 554,047 B1 teaches the growth of SiC crystals by a device using silane and propane as source gases which react in a first reaction zone to form SiC particles to be subsequently evaporated in a lower pressure sublimation zone. The by-products of the crystallization process and the carrier gas are just said to be exhausted through an outlet. In U.S. Pat. No. 5,985,024, filed in 1997, a device is disclosed where silicon vapor is supplied from a heated silicon melt and an hydrocarbon gas such as propane is supplied into the growth zone through a gas supply inlet. In this device, the excess gas downstream of the growing SiC ingot is also just said to be removed from the growth zone by means of a passageway, or outlet channel. As a decreasing temperature distribution is required in or next to the seed holder to promote the growth of the single SiC crystal, it is believed that such passageways will inevitably by subject to a catastrophic blocking by either polycrystalline SiC, pyrolytic graphite or polycrystalline Si deposition. A similar concept is described in U.S. Pat. No. 6,048,398 filed in 1995 where a molten silicon feedstock in combination with a hydrocarbon gas can be used as source gases. The excess gases are exhausted downstream of a seed crystal holder which is rotated and pulled as the single crystal growth proceeds. Despite a beneficial cleaning action of polycrystalline deposits induced by the rotation of the seed holder, such a mechanical cleaning induces stresses in either the rotation mechanism or the seed holder and the elements coming in contact with it. This can lead to mechanical failure of any of the above mentioned parts.
In U.S. patent application no. 2002/0056411 A1, a high temperature vapor deposition apparatus to produce SiC ingots is discussed where the pressure of the gas mixture in the growth region is set higher than that of the exhaust gas mixture to increase the yield of the process. This pressure difference can be achieved by designing the apparatus so that the conductance of the inlet is made higher than the one of the outlet. After a low conductance situated downstream of the single crystal growth zone, the decreased pressure of the exhaust gas mixture causes, at constant temperature, a decrease of the deposition rate of parasitic polycrystalline material. This slows down in any catastrophic blocking along the path downstream of the conductance reducing region of the exhaust. However, as pointed out in the cited application, as the temperature decreases along this downstream path, deposits will again accumulate in a given region, described as gas trap. Preventing these deposits would allow to continue the process for a longer time and to produce longer crystals. Moreover, in this application, the system must be operated at a reduced pressure at least in the downstream part of the conductance reducing region. It can be desirable to instead operate the device at substantially atmospheric pressure, both in the growth zone and in the outlet zone, as this can favor both higher yields and lower cost of the complete system.
It may be noted, that the origin of the problem described above is in a sense fundamental, even if the maximum mass transport of Si-species is arranged at the single crystal growth front. As the growth takes place at high temperature to promote high growth rates and high crystalline quality, to prevent the surface from being graphitized, an amount of Si-vapor at least equal to the equilibrium Si pressure of the heated crystal surface is continuously exhausted downstream of the growth front.